You might check first to see that the problem is really with the camera or the monitor. You can easily get to those and take them down and set them up close together and see if they work fine over a protracted time period. If they do, then you may indeed be looking at a wiring problem. Yes, I know that you do not want to get into this, but it may be necessary. And, yes, pulling new cables will be expensive if others do it for you. Sorry, but that seems to be a fact of life at times.
However, it is also possible that you do not need to have the cables replaced, but you might need some maintenance on the cable connectors. You did say that it was failing intermittently, so the fact that it comes and goes may be saying that there is a problem with either the 12 VDC power going back to the camera, or there is a problem with the video cable bringing the signal up to the monitor. Especially look at the connectors on the outside of the motor home where they attach to the video camera. Look for any sign of corrosion on the connector contact mating surfaces. Check to see that the connectors go together with some pressure and that they are tight, and not loose or worn. You may be able to reform the larger outside body so that it again makes a snug contact with the mating connector. Bend just a little, and do not over-do it, or you may crack the thin sheet metal outside body if they are the old RCA phono plug style video cable connectors. You can replace the connectors on the existing cables if required and the cable itself tests OK. With the cable connectors clean and snug, you can put some silicone grease on the contact mating surfaces to keep water out and reduce corrosion. Try to put some weather sealant over the connectors and that junction when you are putting it back together after your tests are complete, and any required work has been done. That may help to keep the connections reliable and continue working into the future.
If the cables must be replaced, you may be able to tie the new cables onto the old cables and pull the new cables into place as you pull the old cables out, or you can tie a pull line onto the old cable that will go in as you pull the old cable out, and you have the pull line as something to use to pull in the new cable If you can do a good job of making the place where the two cables go together, the old and the new, physically small so that it does not hang up on other things inside as you are pulling them through, then you can pull the new cable in directly, and you will be OK. If it does hang up, do not just yank on it. You may pull apart the cable junction and then have both cables inside with no way to pull the new cable through with the old cable. Then it does become much more difficult. Just pull back a little on the new cable, and try it again. If not, then pull back and take off the new cable and tie a pull line onto the old cable you have just pulled back in, and try again with that method. The best way probably is just to go ahead with the pull line method.
Once you have the new cables installed, check to see that the connectors do go together snugly, and use the same treatment with the silicone grease to prevent oxidation and help keep out water, and weather seal the connector junction.
Again, it is rather likely that you have a connector problem or some similar cable problem that is causing the intermittent. The "bench testing" of the camera and monitor will show whether the problem is in the camera and monitor, or it is still in the cables, and there is more work to do. If it does it on the bench also, then go ahead and either repair or replace the camera or the monitor, depending on which one seems to have the real problem. Here is a place where having a second compatible camera or monitor will help in identifying which one has the problem and needs work. If the camera has the same problem on both monitors, then look at the camera. If the camera works properly with one of the monitors but not the second monitor, then that second monitor may have the problem. If you can get a second camera, and the second camera works with your monitor, then you probably can say there is a problem with your camera. This modular testing process will at least tell you where the problem is.
Latte Land, Washington